The present invention relates to a process for the obtention of alumina and phosphate values by the alkaline decomposition of silica-containing aluminum phosphate ores and, more particularly, it refers to a process for the production of alumina and alkali phosphates by a process including a one-step alkaline decomposition of the aluminum phosphate ores.
For long it has been known that it was not possible to decompose native aluminum phosphates, particularly those containing large proportions of silica, by means of decomposition by a caustic alkali in a one-step process, so as to obtain pure alumina and phosphoric acid or phosphates in an industrially utilizable quality.
It has also been for long known that the presence of phosphoric acid has a disturbing influence in the production of alumina from bauxite and similarly the presence of alumina compounds in an ore constitutes a disturbing factor in the production of phosphate salts from aluminum ores. Also, it is a well known fact that the presence of silica in this type of aluminum phosphate ores has a high disturbing action in the obtention of either alumina or phosphate salts, in view of the fact that the alkaline decomposition of these ores generally tends to dissolve silica which thereby seriously contaminates the liquors of the decomposition, in which the aluminum is contained in the form of alkali aluminate and the phosphorus is contained in the form of soluble alkali phosphates.
It was on account of the above reasons that the most commonly utilized method for the decomposition of aluminum phosphate ores containing large proportions of silica, consisted in effecting a reductive decomposition by means of heat and carbon, under which conditions, phosphorus of phosphoric anhydride are obtained, whereupon the aluminum can be recovered, for instance, as cement, in the residual slags of the reduction furnace.
In view of the fact that the dry process for recovering phosphate values as disclosed above was not able to produce pure alumina for further utilization in the aluminum industry, it has been for long desired to devise aa practical wet process for the alkaline treatment of aluminum phosphate ores, with the purpose of recovering the phosphate values and also the alumina for further utilization in the aluminum industry which up to the present date is based mainly on the existence of bauxite, which is a rather scarce mineral against the enormous reserves of aluminum phosphate ores spread out throughout the world.
To the knowledge of applicant, one of the most efficient processes for the wet alkaline treatment of aluminum phosphate ores was devised some 40 years ago by Hans Huber, as disclosed in U.S. Pat. No. 1,845,876,patented Feb. 16, 1932.Huber describes a process by which the aluminum phosphates can be decomposed by boiling with caustic alkali liquors of a rather low concentration such as those used in the alkaline decomposition of bauxite, whereby a liquor is obtained which contains the tribasic alkali phosphate, caustic alkali and alumina in solution. However, as the concentration of the caustic alkali used by Huber is rather low, in order to crystallize the precipitated alkali phosphate thus formed, it would be necessary to concentrate the solution, and Huber very clearly states that if in this first stage of the process, attempts are made to bring about the complete precipitation of the tribasic alkali phosphate, the objective of recovering the aluminum from said solutions would not be otained, because on the one hand a tribasic alkali phosphate which is highly contaminated with alkali aluminate would be deposited and, on the other hand, the amount of alumina recoverable in a single decomposition would be still very low and thus inadequate, mainly due to the fact that a relatively dilute alkali caustic solution is being used for the alkaline decomposition. The manner in which Huber solves this problem is by recycling the mother liquor obtained after merely cooling the warm solution and separating the small amounts of alkali phosphate from said mother liquor, through the process of alkaline decomposition, with the addition of more caustic alkali and ore. By repeating this operation several times, the mother liquors will be enriched in alumina to enable the latter to be economically precipitated without concentration. Therefore, in accordance with Huber, a number of recirculations are necessary and this multiple-step alkaline decomposition process of aluminum phosphate, which present serious drawbacks, more so if it is considered that the concentration of alkali phosphate in the alumina-enriched solution would still be of around 1%, and this alkali phosphate must be priorly precipitated by the use of lime or the like, whereupon the aluminum can then be recovered from the purified mother liquor as aluminum hydrate.
On the other hand, the efficiency of the process for the recovery of the aluminum is rather low and cannot go beyond around 70 to 90% in accordance with Huber, mainly due to the existence of certain amounts of silica in the starting ore, whereby this process cannot be considered as suitable for recovering alumina and phosphate values from an aluminum phosphate ore which contains relatively high proportions of silica, because said silica is dissolved in the caustic alkali solutions in view of the rather long residence time, and thus would seriously contaminate the aluminum liquors from which the alumina is to be recovered. Therefore, the Huber process presents the serious drawbacks of being a rather complicated process which requires a multiple-step decomposition process and, also, is not suitable for treating aluminum phosphate ores containing relatively high proportions of silica, because the higher the proportion of silica, the lower the recovery of alumina from these mother liquors.
From the above discussion of the prior art processes for the treatment of alkali phosphate ores containing relatively high proportions of silica, it can be concluded that the problem of economically obtaining phosphate values and alumina conjointly from these mineral ores has not yet been solved and that therefore the modern industry still calls for an economical and practical process which may enable the use of the highly abundant aluminum phosphate ores instead of the rather scarce bauxite.